It is desirable for microwave components like bandpass filters and resonators (microstrip, stripline or comb-line) to be tunable over a range of resonant frequencies. The techniques currently used for tuning utilize external elements like varactors or p-i-n diodes. These methods are inherently lossy and cannot maintain a high-Q, or quality factor, over the entire range of tunable frequencies. These limitations can be overcome by using membrane tuning, in which a metallized membrane is suspended over the tunable component, separated by an air gap. The tuning is achieved by changing the capacitive loading of the tunable component by moving the membrane closer or farther from it.
Several methods for membrane deflection have been utilized in the past.
A first type of prior art actuation devices use piezoelectric stacks for membrane deflection. A piezoelectric stack consists of 50-100 piezoelectric layers. In a piezoelectric stack, the thickness (T) of the layer is usually larger than the lateral dimensions, i.e. length (L) or width (W). The stack shortens or elongates along its thickness upon application of an actuation voltage. The piezoelectric stack needs to be several centimeters tall in order to achieve significant travel range for the membrane. This makes the structure bulky and difficult to integrate with the membrane and circuit. See, for example, “Precise control of small displacements of a stacked piezoelectric actuator by means of layer-by-layer driving” by Kondo, S; Yoshimura, S-I; Saito, N; Tanioka, K; Esashi, M. In Micro Electro Mechanical Systems, 2001. MEMS 2001, 14th IEEE International Conference, pp. 248-251.
A second type of actuation devices use electrostatic techniques for membrane deflection. The electrostatic technique involves deposition of thin metal electrodes on the membrane (top electrode) and substrate (bottom electrode), thereby offering the possibility of miniaturization. However, the technique has a severe limitation on the travel range of the membrane due to a nonlinear increase in the deflection force as the gap becomes small. As a result, the membrane collapses onto the substrate when the separation is about ⅓ of the original gap, preventing accurate control of the membrane position. This severely limits the tuning range of the component, which is strongly dependent on the membrane position as the separation becomes small. See, for example, M. Yamaguchi, S. Kawamura, K. Minami, and M. Esashi, “Distributed Electrostatic Micro Actuator,” in Proc. IEEE Conf. on Micro Electro Mechanical Syst., Fort Lauderdale, Fla., Feb. 7-10, 1993, pp 18-23.
In order to decrease the nonlinear effect and partly resolve this problem, the top and bottom electrode positions can be laterally offset. However, this requires a significantly higher actuation voltage and still does not offer a precise control of the membrane position. Additionally, other aspects of the circuit design may place stringent constrains on the position of the electrodes, which would make this technique unfeasible. See, for example, Elmer S. Hung and Stephen D. Senturia, “Extending the Travel Range of Analog-Tuned Electrostatic Actuators”, in J. Microelectromechanical Syst., vol. 8, no. 4, 1999, pp 497-505.
U.S. Pat. No. 5,994,821 to Imada et al. discloses the use of bimorph cantilevers to achieve flexural vibrations which can be used for light deflection. However, the cantilevers in Imada are primarily one-dimensional structures that require the bonding of two prefabricated piezoelectric layers. Additionally, the process disclosed in Imada is not compatible with wafer level photolithographic techniques. A further disadvantage is that the cantilevers in Imada cannot be designed for use as actuators to produce static membrane deflections, as the force generated is quite small.
The paper “Micromachined 2-D Array Piezoelectrically Actuated Flextensional Transducers” by Gokhan Percin and Butrus T. Khuri-Yakub, 1997 IEEE Ultrasonics Symposium, pp. 959-962, discloses piezoelectric actuators integrated with SiN, polysilicon or aluminum membranes. The structure described is used as an ultrasonic transducer, and gives extremely small static deflections (about 0.02 microns) when a 100 V actuation voltage is applied. This renders the actuator useless for tuning applications, for which much larger (about 20 microns) deflections are desirable. An enlargement of the structure to about 10 mm lateral dimensions does not work, as the membrane material is fragile at those sizes. In contrast, polymide membranes are extremely robust for thicknesses as small as about 4 microns and lateral dimensions of about 20 mm.
U.S. Pat. No. 5,406,233 describes a tunable stripline device using at least one strip conductor and at least one ground plane separated therefrom by a dielectric substrate. A layer of piezoelectric material is positioned adjacent the ground plane and a voltage applied to the piezoelectric layer causes its dimensions to change and provide a changing air gap between the substrate and the ground plane. A first disadvantage is that the movement caused by the piezoelectric layer is unidirectional only. A second disadvantage is that the changing air gap between the substrate and the ground plane is not uniform. A third disadvantage is that the device is not compatible with miniaturization and cleanroom processing techniques, so that custom lateral and vertical dimensions for the actuator cannot be fabricated.